Reactive gas detection in complex backgrounds

ABSTRACT

A differential absorption spectrum for a reactive gas in a gas mixture can be generated for sample absorption data by subtracting background absorption data set from the sample absorption data. The background absorption data can be characteristic of absorption characteristics of the background composition in a laser light scan range that includes a target wavelength. The differential absorption spectrum can be converted to a measured concentration of the reactive gas using calibration data. A determination can be made whether the background composition has substantially changed relative to the background absorption data, and new background absorption data can be used if the background composition has substantially changed. Related systems, apparatus, methods, and/or articles are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional patentapplication Ser. No. 60/923,005 filed on Apr. 11, 2007 and entitled“Detection of Hydrogen Sulfide in Hydrocarbon Backgrounds” which isincorporated by reference herein in its entirety.

FIELD

The subject matter described herein relates to detection of reactivegases, such as for example acidic or caustic gas-phase compounds, in gasmixtures containing a complex mixture of background gases.

BACKGROUND

Measurement of caustic and acidic reactive gases, such as for examplehydrogen sulfide (H₂S), hydrogen chloride (HCl), hydrogen fluoride (HF),hydrogen cyanide (HCN), hydrogen bromide (HBr), arsine (AsH₃), phosphine(PH₃), and ammonia (NH₃), in gas streams containing a complex mixture ofpotentially interfering analytes presents a number of difficulties.Accurate characterization of such analytes can be quite important in awide range of applications, including but not limited to petroleumproduct processing, extraction, transportation, and combustion, andnumerous industrial processes.

The reactive gases discussed herein, as well as other gases with similarproperties, can present significant environmental and human safetyhazards. Hydrogen sulfide, for example, is a gas with a characteristic“rotten egg” odor that is highly flammable in air (in a concentrationrange of approximately 4.3% to 45% by volume). OSHA (Occupational Safetyand Health Administration) regulations consider concentrations of 100ppm H₂S “immediately dangerous to life and health”, while concentrationsgreater than 700 ppm lead to immediate death. NIOSH (National Institutefor Occupational Health) recommends the maximum exposure of humans toH₂S to not exceed 10 ppmv for 10 minutes. HF and HBr have OSHApermissible exposure limits of 3 ppmv while the limits for HCN, PH₃,AsH₃, and NH₃ are 10 ppmv, 0.3 ppmv, 0.05 ppmv, and 50 ppmv,respectively. These gases are common byproducts of many industrialpetrochemical processes including ethylene, propylene, Teflon™,polyvinyl chloride (PVC), nylon, viscose rayon production, and rubberproduction, as well as of de-sulfurization of natural gas and crude oil.Trace amounts as low as several ppbv to a few ppmv of these chemicalscan seriously impede the respective chemical processes and lead todefective plastics and other products.

Hydrogen sulfide also occurs naturally in natural gas, oil, sewage andlandfills. Its release to the environment, or incineration forming andreleasing SO₂, even in small quantities, needs to be prevented. Thisrequires sensitive detection of H₂S in pipelines, petrochemicalprocesses and in and around facilities dealing with this gas. Risingworld wide energy demand and energy prices have driven an increasing useof crude oil and natural gas with very high H₂S concentrations whichplaces ever rising demands on H₂S removal technologies and reliable,accurate, and sensitive detection of H₂S.

Natural gas is an important energy source for industry and personalhomes because of its low cost and widespread availability. Unpurifiednatural gas can contain up to 60% hydrogen sulfide. Even less than 4ppmv amounts of H₂S can lead to corrosion of delivery pipelines, overtime, potentially resulting in serious explosion and leakage hazardswhile necessitating costly replacement of segments of the pipeline. Forreference, downtime for natural gas pipelines can cost upwards ofseveral thousand dollars per second. Hydrogen sulfide must be removedfrom natural gas to prevent pipeline corrosion and the emission ofanother toxic gas, SO₂, which is created during burning. Sensitive, realtime detection of hydrogen sulfide in natural gas is becomingincreasingly more important to facilitate limiting concentrations tobelow the 4 ppm tariff level.

Additional concerns with H₂S in gas streams can arise due to itstendency to degrade or poison catalysts in chemical processes. Suchpoisoning can occur due to deposition of surface sulfur on the metalcomponents of a catalyst body and the substitution of sulfur ions foroxygen ions near the surface of metal oxides. In other cases, sulfur canchemically substitute for oxygen in the surface oxides of a catalyst,thereby creating metal sulfides with reduced activity.

Refinery fuel gas is an important energy source for petrochemicalprocessing plants, generating energy from combustible waste gases whichoccur as byproduct of petrochemical processes, including production ofethylene, propylene and iso-butane, which are the fundamental buildingblocks for all types of plastics. U.S. Environmental Protection Agencyregulations limit hydrogen sulfide levels in fuel gas to less than 160ppm for emission of H₂S and SO₂. Also, the presence of NH₃ in fuel gascan lead to formation and emission of environmentally incompatiblenitrous oxides during combustion.

Conventional techniques for measuring reactive gases rely primarily onthe use of chemical sensors such as lead acetate tape, broadband nondispersive UV photometry, gas chromatographs (GC) or small surface areaelectrochemical sensors, such as metal-oxide semiconductors. Thesetechniques have generally proven unsuitable for on-line or at-line realtime process control and real time hazard prevention. Driftingcalibration, slowness of measurement, sensor saturation, long recoverytimes after sensor saturation and sensor element degradation fromcontaminants in the background gas typically lead to erroneous readingsand can lead to harmful emissions or a failure to detect hazardous orunacceptably high concentrations of reactive gases to go undetected.Such conventional sensors tend to be quite maintenance intensive,requiring frequent replacement of costly consumables such as lamps, GCcolumns, carrier gas for GCs, lead acetate tape or aqueous ammoniasolution. In addition, lead acetate tape analyzers create significantamounts of hazardous, lead containing waste. Analyzers relying uponindirect, UV detection of H₂S create di-ammonium sulfide, (NH₄)₂S, wastewhich can easily release H₂S again. Additional limitations can arisefrom electrochemical sensors' abilities to interact with only a verysmall portion of the gas environment, directly at the location of thesensor element itself. This can potentially lead to mischaracterizationof harmful concentrations, particularly in a less than ideally mixed gasstream. Conventional sensors also tend to be highly sensitive tocondensable contaminants in the background gas stream which can causeerroneous readings and earlier than expected sensor failure.

Gas chromatographs (GCs) are being used, generally delivering accurateresults albeit at very high initial, system and infra structure cost,high consumables cost for carrier gas and separation columns and highongoing maintenance cost, providing measurement cycle times in the orderof several minutes. This approach is generally too slow and does notallow real time process control feedback and detection that caneffectively be used to prevent or reduce harmful H₂S concentrations,particularly in gas streams with rapidly varying compositions.

Attempts have been made to measure H₂S and other reactive gasconcentrations by means of ultraviolet light absorption spectroscopy,using a spectrally broad ultraviolet lamp and a diffraction grating. Thecontinuum-like absorption spectrum of, for example, H₂S in theultraviolet can cause erroneous, ambiguous readings and limitedrepeatability of measurement, especially in interfering background gasstreams, making it unsuitable for precise, real time process control andhazard prevention. To overcome this potential ambiguity of direct UVbased H₂S concentration measurement, some non dispersive, broadband UVinstruments attempt to indirectly determine H₂S concentration byconverting H₂S into (NH₄)₂S and photometrically measuring (NH₄)₂S. Thistechnique heavily relies upon the assumption that the chemical reactionto (NH₄)₂S is complete, without aging effects and temperature influence.Another instrument combines the non-dispersive UV absorption techniquewith gas chromatography columns to separate H₂S from the remainder ofthe gas stream, suffering from the same inadequacies as a GC.

HCl, HF, HBr, AsH₃, and NH₃ are also common in a variety of industrialand petrochemical processes and applications, either as chemical processbyproducts or in feed streams. HCl, HBr, and HF are corrosive gases,particularly in the presence of any moisture. Among other possiblesources, they can be used in various applications for production ofplastics and polymers, including PVC, Teflon and nylon, and can also bebyproducts of petroleum cracking, especially in alkylation processes.Ammonia is a caustic gas with a characteristic pungent odor that oftenserves as a precursor to foodstuffs and fertilizers and somepharmaceuticals. Arsine and phosphine have proven detrimental topolymerization reactions, which create the plastic feed stock, even invery low ppb level concentrations. Various wet chemical detectiontechniques or specialized gas chromatographs are available forquantifying HCl, HBr, HF, HCN, AsH₃, PH₃, and NH₃, but like H₂S, realtime, accurate, robust, and low maintenance detection and quantificationmethods for use in industrial processes have been lacking.

SUMMARY

In a first implementation, a method includes determining a firstmeasured concentration of a reactive gas in a first sample of a gasmixture. The gas mixture includes the reactive gas at a reactive gasconcentration and a first background composition that contributesspectral interference that hampers direct spectroscopic measurement ofthe reactive gas concentration in the gas mixture. The first measuredconcentration is determined by generating a first differentialabsorption spectrum by subtracting a first background absorption dataset from a first sample absorption data set collected for the firstsample and converting the first differential absorption spectrum to afirst measured concentration of the reactive gas in the gas mixtureusing calibration data. The first background absorption data setincludes data characteristic of absorption characteristics of the firstbackground composition. A selection is made whether to use the firstbackground absorption data set or a second background absorption dataset to determine a second measured concentration of the reactive gas ina second sample of the gas mixture based on whether a correlation of thefirst background absorption data set to the first background compositionis within a pre-defined tolerance. The second measured concentration isdetermined by generating a second differential absorption spectrum bysubtracting either the first background absorption data set or thesecond background absorption data set from a second sample absorptiondata set for a second sample of the gas mixture and converting the firstdifferential absorption spectrum to the second measured concentration ofthe reactive gas in the gas mixture using the calibration data.

In a second implementation, a system can include a wavelength scannablelaser operating in a scan range that includes a target wavelength, adetector positioned to receive and quantify light intensity from thescannable laser, a sample cell having an interior volume disposed suchthat light from the tunable diode laser passes through at least part ofthe interior volume before the light is received by the detector, amemory that includes or stores a background absorption data set andcalibration data, and a processor. The processor controls the tunablediode laser and receives a sample absorption data set from the detector.The sample absorption data set is collected for a gas mixture in theinterior volume. The gas mixture includes a reactive gas and abackground composition that contributes spectral interference thathampers direct spectroscopic measurement of the reactive gasconcentration in the gas mixture. The processor generates a differentialabsorption spectrum for each sample absorption data set by subtractingthe background absorption data set from the sample absorption data setand converts the differential absorption spectrum to a measuredconcentration of the reactive gas in the gas mixture using calibrationdata. The processor further determines whether the backgroundcomposition has substantially changed relative to the backgroundabsorption data set by analyzing the differential absorption spectrumand switching to a new background absorption data set if the backgroundcomposition has substantially changed.

In optional variations, one or more of the following additional featurescan be included. A second background absorption data set can be usedthat differs from the first background absorption data set to generatethe second differential absorption spectrum if the first backgroundcomposition has a substantially different absorption profile relative tothe first background absorption data set. A second background absorptiondata set that is identical to the first background absorption data setcan be used to generate the second differential absorption spectrum ifthe first background composition has a substantially similar absorptionprofile relative to the first background absorption data set.

The first differential absorption spectrum can include a first regionwith a first spectral response characteristic of the concentration ofthe reactive gas and a second region with a second spectral responsecharacteristic of a correlation between the first background compositionand the first background absorption data set. The first region and thesecond region would not substantially overlap, and the determining caninclude comparing the correlation to a predetermined threshold. Thefirst region can optionally include wavelengths within approximately ±1cm-1 of the target wavelength. The first sample absorption data set canbe collected using a scannable laser source operating in a scan rangethat includes the target wavelength. The scannable laser source canoptionally include a tunable diode laser or a quantum cascade laser.

The background absorption data set can optionally be collected for abackground sample prepared from the gas mixture using the scannablelaser in the scan range. The preparing of the background sample canoptionally include treating a volume of the gas mixture to reduce aconcentration of the reactive gas without substantially altering thebackground composition in the background sample. The treating of the gasmixture can optionally include passing the gas mixture through ascrubber material that converts molecules of the reactive gas to anon-gaseous state. The scrubber material can optionally include one ormore of metal oxides, cupric dicarbonate, solid and liquid-phaseinorganic and organic acids and bases, metal oxide and copper oxide nanoparticles suspended on larger grain size carrier particles, solid statescrubbers, liquid scrubbers, amine solutions, aqueous ammonia solutions,and aqueous solutions of strong acids or bases. The first sampleabsorption data set and the first background absorption data set caninclude be collected alternately in a single sample cell. Alternatively,the first and second sample absorption data sets and the first andsecond background absorption data set can optionally be recorded in asample cell and a background sample cell, respectively, withsubstantially identical optical path lengths.

The first and the second background absorption data sets can optionallyinclude empirically obtained historical absorption data for a first anda second characteristic background composition. The reactive gas canoptionally be selected from a group consisting of hydrogen sulfide,hydrogen chloride, hydrogen fluoride, hydrogen bromide, hydrogencyanide, arsine, phosphine and ammonia and the target wavelength is oneat which the reactive gas has an absorbance selected according to afigure of merit greater than 1×10⁻⁶. The background composition canoptionally include one or more of a group consisting of natural gas,alkanes, refrigerants, olefins, hydrogen, nitrogen, oxygen, chlorine,carbon dioxide, ammonia, water, carbon monoxide, hydrocarbons,hydro-fluorocarbons, hydro-chlorocarbons, and hydrofluorochlorocarbons.The scan range can optionally include wavelengths within approximately±2 cm-1 or less of the target wavelength.

Articles are also described that comprise a tangibly embodiedmachine-readable medium operable to cause one or more machines (e.g.,computers, etc.) to result in operations described herein. Similarly,computer systems are also described that can include a processor and amemory coupled to the processor. The memory can include one or moreprograms that cause the processor to perform one or more of theoperations described herein.

The subject matter described herein provides many advantages. Opticalabsorption spectroscopy can be used to detect reactive gases, includingbut not limited to H₂S, HCl, HF, HBr, HCN, AsH₃, PH₃ and NH₃. Light froma source such as a laser or any other suitable, spectrally narrow lightsource is detected by a detector after passing through a gas sample. Bymonitoring the amount of light absorbed by the sample, at specificwavelengths, the concentration of the target gas can be accuratelydetermined. Among other possible benefits, improved signal to noiseratio and lowered minimum detection limits can be achieved by reducingsystem background noise, such as that originating from electroniccircuitry, optical fringes, ambient noise and laser noise, by combining2 f wavelength modulation spectroscopy and phase sensitive lock-inamplification with differential spectroscopy.

The details of one or more variations of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, show certain aspects of the subject matterdisclosed herein and, together with the description, help explain someof the principles associated with the disclosed embodiments. In thedrawings,

FIG. 1A and FIG. 1B are charts showing sample absorbance spectra for acomplex gas mixture containing a reactive gas with and withoutscrubbing;

FIG. 2 is a process flow diagram illustrating a method for determiningreactive gas concentrations in complex backgrounds;

FIG. 3 is a chart showing a sample differential spectrum for a reactivegas in a complex gas mixture;

FIG. 4 is a schematic diagram illustrating a first system for analyzingreactive gas concentrations in complex gas mixtures;

FIG. 5 is a schematic diagram illustrating a second system for analyzingreactive gas concentrations in complex gas mixtures;

FIG. 6 is a chart showing transmission spectra of H₂S and pure methaneat different pressures with different absorption path lengths;

FIG. 7 is a chart showing absorption spectra of H₂S;

FIG. 8 is a chart showing differential absorption spectra for H₂S atdifferent concentrations; and

FIG. 9 is a chart showing calibration curves for H₂S.

DETAILED DESCRIPTION

The subject matter described herein relates to the detection of traceamounts of reactive gases, such as for example H₂S, HCl, HF, HBr, HCN,AsH₃, and NH₃, in various infrared absorbing background gases andmixtures of those gases, including, but not limited to, methane, ethane,propane, pentane, hexane, ethylene, propylene, iso-butane, ambient air,nitrogen, oxygen, hydrogen, CO, CO₂, chlorine, fluoro-carbons,chloro-carbons, chloro-fluoro-carbons, natural gas and refinery fuelgas, and other non-infrared absorbing gases such as noble gases, O₂, N₂,H₂, and Cl₂. Sensitive detection h of reactive gases like H₂S, HCl, HF,HBr, HCN, AsH₃, PH₃ and NH₃ in the environment, inside petrochemicalproduction, natural gas pipelines and processes such as Clausde-sulfurization plants and waste incinerators is important to preventoccupational, environmental and industrial safety hazards, harmfulaccidents and costly repairs, especially of pipelines used fortransportation of natural gas and petrochemical process gases.

A common challenge with optical absorption spectroscopy arises from thefact that multiple constituents in a sample can absorb at substantiallythe same wavelength over wide spectral ranges. This is especially truefor low concentrations of reactive gases such as H₂S, HCl, HF, HBr, HCN,AsH₃, PH₃ and NH₃ in a background of hydrocarbon and fluoro- orchloro-carbon gases. As an example, natural gas used for burning, whichis typically composed of greater than 80% methane (CH₄), containshydrogen sulfide at typically less than 4 ppmv, as per distributiontariff. Refinery fuel gas, which can contain between 30% and 60% methaneand other hydrocarbon gases, has to maintain H₂S concentration at lessthan 160 ppmv to comply with EPA and other emission regulations.Conventional, spectrally broad spectroscopic methods (i.e., non-laserbased) are generally not suitable for measurement of hydrogen sulfide ina CH₄ or hydrocarbon gas background because the absorption by CH₄, andother hydrocarbon gases which are present in much larger quantities,completely obscures the much weaker absorption by H₂S at all wavelengthsin the visible and infrared region. Similar overlap can be observed forother reactive gases such as for example HCl, HF, HBr, HCN, AsH₃, PH₃and NH₃. This type of spectral overshadowing of the target analyteabsorption can occur with substantially all hydro-, fluoro, andchloro-carbon gases, including but not limited to methane, ethane,propane, butanes, pentanes, hexanes, septane, octane, nonane, decane,ethylene, propylene, iso-butane and mixtures thereof.

In one implementation, sensitive reactive gas detection can be achievedby overcoming spectral overshadowing of one or more reactive gasabsorption lines by the background gas using differential tunable diodelaser (D-TDL) absorption spectroscopy. Laser absorption of a gas streamor sample is measured in a scan range that includes the wavelength of aselected reactive gas absorption line to generate a sample absorptiondata set. A background absorption data set is subtracted from the sampleabsorption data set to generate a differential absorption spectrum fromwhich the reactive gas concentration can be obtained using calibrationdata from one or more predetermined calibration values.

The background absorption data set can in some implementations be one ofa group of background absorption data sets that are pre-loaded in amemory that is accessible by a processor that converts the absorptiondata to concentration values. In these implementations, a backgrounddata set that is expected to be most representative of the actualbackground composition of the gas mixture is selected.

In other implementations, a background absorption data set can begenerated in real or near-real time by treating a sample of the gasmixture to reduce the reactive gas concentration while leaving thebackground composition of the gas mixture substantially unchanged. Thistreating can be accomplished in some examples by passing a sample volumeof the gas mixture through a scrubber device that contains a materialthat preferentially reacts with the reactive gas to substantiallyconvert the reactive gas to a non-gaseous state. Laser absorption of thebackground sample is measured in the same scan range that includes thewavelength of the selected reactive gas absorption line to generate abackground sample absorption data set. In this manner, the backgroundcomposition of the gas mixture that contributes spectral interference isdirectly measured and accounted for in the differential absorptionspectrum, thereby permitting accurate extraction of the spectralresponse attributable to the reactive gas even in gas mixtures withdynamically variable background compositions.

A new background sample can be prepared and analyzed periodically basedon a preset or user selectable interval, continuously in parallel withan unscrubbed sample, or via an automated initiation process via whichthe differential absorption spectrum is analyzed to ascertain whetherthe current background absorption data set adequately characterizes thecurrent background composition of the gas mixture. This automaticinitiation is discussed in greater detail below.

The scrubber material can be selected from a range of options, includingbut not limited to metal oxides, cupric dicarbonate, liquid and solidinorganic and organic acids and bases, solid state scrubbers, liquidscrubbers, amine solutions, aqueous ammonia solutions, and aqueoussolutions of strong acids or bases. In some implementations, thescrubber material can include metal oxide nano particles, such as forexample copper oxide nano particles that are suspended on larger grainsize carrier particles. Examples of this scrubber material are discussedin greater detail in co-pending and co-owned provisional application forU.S. patent No. 60/968,846, the disclosure of which is incorporated byreference herein.

FIG. 1A and FIG. 1B show graphs of absorbance vs. wavelength for twocomplex gas mixtures. The first gas mixture, whose absorbance spectrum102 is shown in FIG. 1A, contains hydrogen sulfide in addition to abackground composition that contains one or more gases with absorbancefeatures that might interfere with spectral analysis of hydrogensulfide. A substantial absorption peak 104 corresponds to a hydrogensulfide absorption feature. The second gas mixture, whose absorbancespectrum 106 is shown in FIG. 1B is identical to that of the first gasmixture with the exception of the hydrogen sulfide which has beenremoved or at least substantially reduced in concentration. Although thepeak 110 corresponding to the location of the hydrogen sulfide peak 104in FIG. 1A is not as pronounced, substantial absorption nonethelessremains. Particularly in hydrocarbon mixtures with varying composition,the strength of this interfering absorbance can be significant andnon-constant and therefore quite difficult to correct for viaconventional differential spectroscopy methods.

In various implementations, the current subject matter can be used toaccurately measure reactive gas concentrations in gas mixtures havingbackground compositions that include infrared absorbing gases and anymixture thereof, including but not limited to natural gas, methane,ethane, propane, butanes, hexanes, septane, octane, nonane, decane,ethylene, propylene, iso-butane, fluorocarbons, chlorocarbons,fluorochlorocarbons, CO, CO₂, and H₂O. The background composition canalso include gases which do not absorb in the infrared, including butnot limited to noble gases, O₂, N₂, H₂ and Cl₂.

FIG. 2 is a process flow chart 200 describing a method that can be usedin some implementations to accurately quantify reactive gasconcentrations in a gas mixture having a dynamic, complex backgroundcomposition. At 202, a first measured concentration of a reactive gas ina first sample of a gas mixture that contains a first backgroundcomposition is determined by generating first differential absorptionspectrum at 204. The first differential absorption spectrum can begenerated by subtracting a first background absorption data set from afirst sample absorption data set. The method can optionally includecollecting the first background absorption data set for a firstbackground sample at 206 and collecting the first sample absorption dataset for a first sample at 210. The first background sample includes datathat are characteristic of the absorption characteristics of the firstbackground composition. At 212, the first differential absorptionspectrum is converted to the first measured reactive gas concentrationusing calibration data. The calibration data can be obtained byanalyzing one or more standard samples having a known concentration ofthe reactive gas or gases of interest and quantifying the differentialspectral response for the standard sample or samples. In someimplementations, standard samples with different background compositionscan be used to improve accuracy of the measured reactive gasconcentrations. Other types of calibration data could also be used.

Based on an analysis of the first differential absorption spectrum, aselection is made at 214 whether to use the first background absorptiondata set for the next sample or to switch to a second backgroundabsorption data set. This selection can in some implementations includeanalyzing a second region of the first differential absorption spectrumthat is distinct from a first region of the first differentialabsorption spectrum in which a substantial reactive gas absorptionfeature occurs. FIG. 3 shows a chart 300 of a differential absorptionspectrum 302 resulting from the subtraction of the absorption curve ofFIG. 1B from that of FIG. 1A. In one example, the first region 304 mightbe the part of the first differential absorption spectrum lying within arange of ±1 cm⁻¹ on either side of the target wavelength 306 which canat least partially coincide with a substantial absorption feature 308 ofthe reactive gas of interest. The second region 310 could be theremainder of the first differential absorption spectrum. The secondregion 310 can optionally be analyzed to determine how well the firstbackground absorption data set represents the actual absorption due tothe first background composition. This can be accomplished using one ofa number of mathematical correlation functions or the like. In effect,this process examines how well subtraction of the first backgroundabsorption data set from the first sample absorption data set leads to aconstant observed absorption in the second region of the firstdifferential absorption spectrum. If the resulting first differentialabsorption spectrum is not substantially constant in the second region,this can be taken as an indication that the background composition ofthe gas mixture has changed relative to the characterization of thebackground composition that is presented by the first backgroundabsorption data set.

Continuing with FIG. 2, a second measured concentration of the reactivegas in a second sample of the gas mixture that contains a secondbackground composition is determined by generating first differentialabsorption spectrum at 216. The second differential absorption spectrumcan be generated by subtracting a second background absorption data setfrom a second sample absorption data set at 220. The method canoptionally include collecting the second background absorption data setfor a second background sample at 222 (if a second background absorptiondata set is determined to have been needed at 214) and collecting thesecond sample absorption data set for a second sample at 224. The secondbackground sample data set includes data that are characteristic of theabsorption characteristics of the second background composition. At 226,the second differential absorption spectrum is converted to the secondmeasured reactive gas concentration using the calibration data.

The background sample data sets discussed above can be generated in realor near real time one of the systems discussed below or some otherconfiguration that allows a reproducible measurement of a gas mixtureboth with and without the reactive gas of interest. In some optionalimplementations, a background sample is collected periodically at somepreset interval to routinely verify that the background composition ofthe gas mixture is well characterized by the background data set beingused to generate the differential absorption spectrum for a givensample. This periodic feature can be used either in conjunction with theautomated system described above or as a stand alone feature in whichthere is no automated process for determining if a new background dataset is needed. In other implementations, a live background data set isnot collected. Instead an archived background absorption data set isused based on one or more measurable characteristics of the gas mixture.For example, the background composition of a natural gas stream could beperiodically determined using a gas chromatograph, and a backgroundabsorption data set collected for a sample having the identifiedcomposition can be used.

Alternatively, the background sample absorption data set can be selectedbased on a known process stream, based on real time measurements of asingle component of the gas mixture that is used as a surrogate for theoverall gas mixture composition. These same methods could be also beused to identify when a new real time or near real time measurement ofthe background composition should be performed. Other methods andsystems for identifying a most representative background data set froman archive and/or determining that a new background sample should beanalyzed can also be used. For example, in implementations where thesecond background absorption data set is selected 214 from an archive ofhistorical or empirically generated background data sets, the newbackground absorption data set can optionally be selected using thefirst sample absorption data set to generate one or more testdifferential absorption spectra with one or more candidate archivedbackground absorption data sets. The candidate background sampleabsorption data set that generates a test differential absorptionspectrum showing the best correlation in the second region can beselected as the second background sample absorption data set.

FIG. 4 and FIG. 5 illustrate sample analyzers that can be used to detectand quantify reactive gas concentrations in gas mixtures with complexbackground compositions. FIG. 4 depicts an analyzer 400 with a dual beamarrangement in which the beam 402 from the light source 404 is split bya beam splitter 406 and mirror 408 into a first beam 410 and a secondbeam 412 that passes through gas held in a first 414 and a second 416sample cell, respectively. The first sample cell 414 contains a firstsample of the gas mixture that is treated to be a background sample asreferred to in FIG. 2. The background sample can be prepared by removingor reducing the reactive gas concentration (for convenience, the processof reducing the concentration of the reactive gas is referred to as“scrubbing” for purposes of this document). The second sample cell 416contains a second sample of the gas mixture that has not been scrubbed.FIG. 5 shows an alternative analyzer 500 with a single beam, singlesample cell arrangement in which the first sample and the second samplealternatively and sequentially enter the sample cell 502 where they areilluminated by the beam 504 from the light source 506.

More specifically, with reference to the analyzer 400 shown in FIG. 4,the first beam 410 is directed through the first sample cell 414containing the first sample which has been scrubbed by passing itthrough a scrubber 420. The second beam 412 is directed through a secondsample cell 416 of identical optical path length to the first samplecell 414. The second sample cell 416 contains the second sample whichhas not been scrubbed. As such, the second sample contains componentsfound in the first sample (e.g. the background sample) in addition tothe reactive gas at the concentration present in the gas mixture. Inoperation, gas flowing into the analyzer 400 is split between the twofirst 414 and the second 416 sample cells. This can be accomplished by aflow divider 422 or other equivalent apparatus for dividing gas flowbetween two channels. Gas flowing to the second sample cell 416 passesthrough the scrubber 420 that reduces the reactive gas concentrationfrom the gas mixture to produce the first sample that is the backgroundsample.

The scrubber 420 can be any device or process that reduces theconcentration of the reactive gas in the gas mixture, including but notlimited to those mentioned above. The scrubber 420 is advantageouslychosen to not substantially affect the concentration of the othercomponents of the gas mixture by more than 5% by volume. Gas flowing tothe second sample cell 416 does not pass through the scrubber 420.

The first 410 and second 412 split beams pass into the first 414 andsecond 416 sample cells respectively. Depending on the configuration ofthe analyzer 400, the incident light can pass through first windows 424as shown in FIG. 4. The gas in each sample cell can absorb some fractionof the beam intensity, and the first 410 and second 412 light beams thenimpinge upon a first 426 and a second 430 detector respectively. Thefirst 426 and second 430 detectors can each be a device that quantifiesan intensity of light that is incident on a surface or aperture of thedetector. In some implementations, the detector 426, 430 can be,photodetectors, including but not limited to an Indium gallium arsenide(InGaAs) indium arsenide (InAs), silicon (Si), germanium (Ge)photodiode, a mercury-cadmium-telluride (MCT), lead-sulfide (PbS)photodetector, or another photodetector which is sensitive to light inthe 400 to 50000 nm wavelength regions.). Depending on the configurationthe beams can pass through second windows 432 to exit the first andsecond sample cells. The example illustrated in FIG. 4 depicts the firstand second sample cells as single pass configurations in which the beamsenter the respective sample cells through first windows 424, passthrough the gas contained in each sample cell, and exit the respectivesample cells through second windows 432. Other configurations are withinthe scope of the disclosure, as discussed below.

The first detector 426 quantifies the intensity of the first beamimpinging upon it, and thus passing through the first sample cell 414,as a function of wavelength. Likewise, the second detector 430quantifies the intensity of the second beam impinging upon it, and thuspassing through the second sample cell 416, as a function of wavelength.In this manner, the first detector 426 quantifies the transmittedintensity for the first sample, in this example the scrubbed backgroundsample, and the second detector 430 quantifies the transmitted intensityfor the second sample, which has not been scrubbed. Data from the firstdetector 426 and the second detector 430 is passed to a control unit434, such as for example a microprocessor, which records and/orprocesses data from the detector to generate a differential spectrumfrom which the reactive gas concentration in the second sample can becalculated. The concentration of the reactive gas is dependent on themole fraction of reactive gas molecules as well as the temperature andpressure of the gas mixture being measured. As such, the temperature andpressure in the first 414 and second 416 sample cells can be monitoredand/or controlled.

To account for detector drift and other potential measurement artifacts,some variations can periodically record an absorption spectrum for eachsample cell with no gas to determine the detector's dark current “zero”or to periodically reverse the flows such that the first sample cell 414is supplied with unscrubbed gas and the second sample cell is suppliedwith the scrubbed, background sample.

The light source 404 can, in some implementations, operate at aspectrally very narrow wavelength substantially corresponding to areactive gas absorption line where minimal absorption occurs by thebackground composition of the gas mixture, thereby minimizing theeffects of interference due to the extremely high spectral purity of thelaser (narrow line width). The current system can incorporate a laser asits light source, emitting in the wavelength range between 400 nm and20,000 nm. Tunable diode lasers emitting light within the wavelengthrange from 400 nm to 3000 nm can be utilized. In addition, quantumcascade lasers (such as those described by J. Faist, F. Carpasso, D. L.Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, Appl. Phys. Lett.72, 680 (1998), the contents of which are hereby incorporated byreference) emitting light in the wavelength range from 4000 nm to 20,000nm can also be utilized. Alternately, the spectrally narrow light sourcecan also be constructed by nonlinear difference and sum frequency mixingof suitable lasers. However, nonlinear frequency mixing can be opticallycomplex and too expensive for practical commercial applications.Alternatively, a color center laser can be utilized, but such lasers arenot always suitable for use in commercial field instrumentation due totheir relatively large physical size, high power consumption, highmaintenance requirements, need for cryogenic cooling, and cost.

The light source 404 can optionally be a single frequency diode laser orother light source that emits at the target wavelength and that isscannable over a frequency or wavelength range in which the targetwavelength is found. Illustrative examples of target wavelengths aredisclosed below. Other wavelengths where the reactive gas molecule has astrong absorption line and the interference absorptions from other gasspecies in the background composition of the gas mixture, such as forexample CH₄, H₂O and CO₂, are relatively weaker can also be used.Alternatively the light source 404 can optionally be a quantum cascadelaser, or the like. In some variations, the wavelength of a tunablediode laser light source 404 can be scanned across the reactive gasabsorption feature by varying the injection current while keeping thelaser temperature constant. The laser temperature can in someimplementations be controlled by placing the laser in intimate contactwith a thermoelectric cooler (Peltier cooler) whose temperature ismeasured with a thermistor and controlled by a feedback circuit.

Due to the removal of the reactive gas in the background sample, thelight source 404 can operate at any reactive gas absorption linewavelength between 400 nm and 20,000 nm. In one implementation, lasersin the economically advantageous telecommunications wavelength bandbetween 1500 nm and 1610 nm, including but not limited to 1567 nm,1569.9 nm, 1574.5 nm, 1576.3 nm, 1578.1 nm, 1581.3 nm, 1582.1 nm, 1589.2nm, 1589.8 nm, 1590 nm, and 1601.3 nm can be utilized for analysis ofH₂S.

FIG. 5 depicts an analyzer 500 with a single-beam arrangement. A firstsample that has been scrubbed and a second, non-scrubbed sample arealternately illuminated by the beam 504 from the light source 506 (whichcan have the same characteristics as light source 404 in FIG. 4) in asample cell 502. Spectra are recorded individually for the first sample,which is the scrubbed background sample, and the second sample, which isnot scrubbed. For a flow system, this process can be performedcontinuously and sequentially for multiple samples and multiplebackground samples. The analyzer 500 in FIG. 5 includes a scrubber 510that can be placed in series with the gas inlet 512 to the sample cell502 by, for example a pair of multi-way valves 514 which can optionallybe solenoid valves or pneumatically operated valves. As noted above, thescrubber 510 can be any device or process that reduces the concentrationof the reactive gas in a complex background. As with the system shown inFIG. 4, the scrubber 510 is advantageously chosen to not substantiallyaffect the concentration of the other components of the sample gasmixture. The second sample is not passed through the scrubber 510 and assuch retains the hydrogen sulfide concentration that is present in thegas being measured.

In operation of the analyzer 500 shown in FIG. 5, gas is alternativelyconveyed to the sample cell inlet 512 either directly or via thescrubber 510 by appropriate operation of the two way valves 514. Thedetector 516 quantifies the intensity of the beam 504 impinging upon it,and thus passing through the sample cell 502, as a function ofwavelength. Thus, when the first sample, which passes through thescrubber to reduce its reactive gas concentration, is in the sample cell502 the detector 516 quantifies the transmitted intensity for the firstsample, in this example the scrubbed background or reference gas. Thedetector 516 quantifies the transmitted intensity for the second sample,containing the original reactive gas concentration, when gas flowsdirectly to the sample cell without passing through the scrubber 510.

The sample beam can optionally enter the sample cell through an inputwindow 520 and exit the cell though an exit window 522. Alternativesample cell configurations, such as those discussed above in regards toFIG. 4, are also within the scope of this disclosure. Gas exits thesample cell 502 via the exhaust outlet 524. Intensity data from thedetector 516 are passed to a data analysis device 526, such as forexample a microprocessor. The data analysis device 526 records and/orprocesses data received from the detector for the first sample and thesecond sample to generate a differential spectrum from which thereactive gas concentration in the second sample can be calculated. Theconcentration of reactive gas is dependent on the mole fraction ofhydrogen sulfide molecules as well as the temperature and pressure ofthe gas being measured. As such, the temperature and pressure in thesample cell 502 can be monitored and/or controlled.

As noted above, a first sample and a second, scrubbed sample of a gasare illuminated by a laser light source. The path length of the samplecell can be varied depending on the strength of the specific absorptionline of interest or the magnitude of the difference between theabsorption line of interest and interfering absorption lines from othergas species present. A cell of insufficient length can not providesufficient sensitivity while one of excessive length can absorb theentirety of the incident light such that no measurable signal reachesthe detector (a situation called saturation). Other aspects of a similaranalyzer and related techniques are described in U.S. patent applicationSer. No. 11/715,599, filed on Mar. 7, 2007, and entitled “MeasuringWater Vapor in Hydrocarbons,” the disclosure of which is incorporatedherein by reference.

To achieve longer optical path lengths without the use of extremely longsample cells, sample cell configurations within the scope of thisdisclosure can also include the use of one or more mirrors or reflectiveor refractive optical elements to route the beam such that the beampasses through the sample contained in the sample cell two or moretimes. In such a multipass configuration, the beam can enter and exitthe cell through the same window or through different windows. In someimplementations, windowless sample cell configurations can be utilizedin which, for example, the laser source and/or the detector arecontained within the sample cell. In some variations, a Herriott cell isutilized to increase effective path length. Alternatively, the lightpath can be a straight line between light source and detector in an openenvironment or inside a transport pipe or container for the sample gas.

Spectral overshadowing of the reactive gas absorption lines by thebackground gas can be overcome by operating the measurement at lowpressure. At reduced pressure, spectral line broadening effectsdiminish, leading to spectral sharpening of absorption lines of everyspecies in the gas mix. Depending upon the background composition of thegas mixture, this reduces or eliminates spectral overlap between thereactive gas and the background components of the gas mixture, enablingsensitive reactive gas detection. Suitable lines can in someimplementations fulfill a figure of merit (FOM) of >1×10⁻⁶, at therespective working pressure. The FOM is defined as the absorption of 1ppmv of the reactive gas divided by the total background gas absorptionat the chosen wavelength. Low pressure TDL can be used to enhancereactive gas detection in all infrared absorbing gases. Low pressuremeasurements can be combined with the afore described differential TDLtechnique to further enhance sensitivity and broaden applicability tobackground gas streams.

In some variations, the system measures reactive gas in symmetricallydiatomic gases which exhibit no interfering infrared backgroundabsorption. Such diatomic gases include but are not limited to air, O₂,H₂, N₂, and Cl₂. For H₂S, HCl, HF, HBr, HCN, AsH₃, PH₃, and NH₃, nearlyevery absorption line between 400 nm and 20,000 nm fulfills thedetection requirements. The system can also measures reactive gas innoble gases which exhibit no interfering infrared background absorption.Such noble gases include Ne, Ar, Kr, Xe and Rd. For these gases, everyH₂S, HCl, HF, HBr, HCN, AsH₃, PH₃, and NH₃ absorption line between 400nm and 20,000 nm fulfills the detection requirements.

For H₂S detection in air or in CO, CO₂ or any other gases exhibitinginfrared absorption, H₂S absorption lines between 400 nm and 20,000 nmexhibiting minimum overlap with the background gas absorption aresuitable. The specific absorption transitions for measurement of H₂S,HCl, HF, HCN, HBr, AsH₃, PH₃, and NH₃ in various background gases aresummarized in Tables 1-8, respectively. However, it will be appreciatedthat other wavelength ranges can be utilized provided that the reactivegas molecules absorb light at a greater level than do background gasmolecules. Line selection can be facilitated using a Figure of Merit(FOM), which is defined as the absorption of 1 ppmv of the reactive gasdivided by the total background absorption. Reactive lines satisfying anFOM of >1×10⁻⁶ may be suitable for sensitive detection according to thecurrent subject matter.

TABLE 1 Illustrative absorption transitions for measurement of H₂S1567.0 nm 1569.9 nm 1574.6 nm 1576.3 nm 1578.1 nm 1581.3 nm 1582.1 nm1589.2 nm 1589.8 nm 1590.0 nm 1590.2 nm 1590.6 nm 1591.1 nm 1601.3 nm1919.1 nm 1919.3 nm 1928.0 nm 1944.6 nm 2599.1 nm 2604.7 nm 2639.6 nm2650.1 nm 3718.7 nm 3730.0 nm 4049.2 nm 4129.3 nm 7460.5 nm 7601.7 nm7734.6 nm 7893.3 nm

TABLE 2 Illustrative absorption transitions for measurement of HCl1727.0 nm 1730.2 nm 1733.9 nm 1737.9 nm 1742.4 nm 1747.2 nm 1752.5 nm1777.8 nm 1785.2 nm 1793.0 nm 1801.2 nm 1810.0 nm 1819.1 nm 3335.4 nm3354.6 nm 3374.6 nm 3395.7 nm 3417.8 nm 3440.9 nm 3490.3 nm 3516.6 nm3544.1 nm 3572.8 nm 3602.6 nm

TABLE 3 Illustrative absorption transitions for measurement of HF 1264.3nm 1268.3 nm 1273.0 nm 1278.1 nm 1283.9 nm 1297.1 nm 1304.5 nm 1312.6 nm1321.3 nm 2395.8 nm 2413.8 nm 2433.1 nm 2453.8 nm 2475.9 nm 2499.4 nm2550.8 nm 2578.8 nm 2608.5 nm

TABLE 4 Illustrative absorption transitions for measurement of HCN1527.4 nm 1528.0 nm 1528.6 nm 1538.9 nm 1539.7 nm 1540.5 nm 1844.7 nm1845.5 nm 1846.4 nm 1861.4 nm 1862.6 nm 1863.7 nm 2483.2 nm 2484.9 nm2497.5 nm 2508.7 nm 2512.6 nm 2518.5 nm

TABLE 5 Illustrative absorption transitions for measurement of HBr1337.6 nm 1339.1 nm 1341.0 nm 1342.8 nm 1345.1 nm 1360.4 nm 1953.9 nm1957.8 nm 1962.0 nm 1966.9 nm 1972.0 nm 1977.1 nm 1995.8 nm 2002.8 nm2010.3 nm 2018.4 nm 2026.5 nm 2035.2 nm

TABLE 6 Illustrative absorption transitions for measurement of AsH₃2162.1 nm 2165.8 nm 2168.6 nm 2172.4 nm 2176.4 nm 2192.2 nm 2201.7 nm2215.4 nm 2220.0 nm 2227.3 nm 2818.4 nm 2826.5 nm 2851.5 nm 2889.5 nm2900.1 nm 2920.6 nm 2952.3 nm 2975.1 nm 4555.4 nm 4582.8 nm 4597.4 nm4625.9 nm 4641.3 nm 4702.3 nm 4800.4 nm 4814.9 nm 4833.2 nm 4851.9 nm4870.8 nm 4890.2 nm

TABLE 7 Illustrative absorption transitions for measurement of PH₃2366.2 nm 2369.5 nm 2373.0 nm 2376.4 nm 2380.0 nm 2383.8 nm 2400.3 nm2424.8 nm 2427.1 nm 2432.0 nm 2436.9 nm 2438.4 nm 4163.4 nm 4187.7 nm4201.7 nm 4215.2 nm 4226.0 nm 4241.2 nm 4301.1 nm 4311.1 nm 4342.2 nm4386.8 nm 4405.0 nm 4431.9 nm

TABLE 8 Illustrative absorption transitions for measurement of NH₃1512.0 nm 1531.7 nm 1955.3 nm 1958.5 nm 1963.1 nm 1966.2 nm 1981.5 nm1993.3 nm 1997.2 nm 2005.2 nm 2230.1 nm 2239.0 nm 2275.9 nm 2285.2 nm2295.7 nm 2305.5 nm 2896.4 nm 2897.7 nm 2912.1 nm 2913.9 nm 2928.6 nm2946.5 nm 2961.8 nm 2997.3 nm

In various implementations, the current subject matter can be used tomeasure hydrogen sulfide in the near-infrared absorption bands, and morespecifically, at wavelengths such as 1567 nm, 1569.9 nm, 1581.3 nm,1582.1 nm, 1589.3 nm, 1589.6 nm, 1601.3 nm, 1944.6 nm, and 2650.1 nm,where absorption by methane and the infrared-absorbing constituentswithin air (the two main constituents being water and carbon dioxide)are weak. FIG. 6 is a transmission spectrum 600 of hydrogen sulfide 602and pure methane 604 ranging from 3770 to 3778 cm⁻¹. 2. FIG. 7 is agraph 700 showing absorption spectra of H₂S (100 ppm) 702, CO₂ (1%) 704and H₂O (100 ppm) 706. As can be seen, the hydrogen sulfide transition710 at 2.650 um is well isolated from interference of methane, water andcarbon dioxide, thereby allowing hydrogen sulfide to be measured innatural gas, hydrocarbon gas, fluorocarbon gas, hydro-fluorocarbon gasand other background gases.

In some variations, the current system employs harmonic spectroscopy.Harmonic spectroscopy as used in the current system involves themodulation of the TDL laser wavelength at a high frequency (kHz-MHz),detecting the signal at a multiple of the modulation frequency.Detection is performed at twice the modulation frequency, thus the termsecond harmonic spectroscopy is used. Advantages to this techniqueinclude the minimization of 1/f noise, and the removal of the slopingbaseline that is present in TDL spectra. The laser output powerincreases as the laser injection current increases, creating a slopingbaseline for the light detected by the photodetector. Changing the laserinjection current is how the laser is tuned across the absorption line.

In some implementations, direct absorption spectroscopy can be utilized.With such an arrangement, the laser frequency is tuned across theselected absorption transition and the zero-absorption baseline istypically obtained by fitting the regions outside the absorption line toa low-order polynomial. The integrated absorbance is directlyproportional to absorbing species concentration as well as the linestrength of the transition.

Cavity-ring down spectroscopy can also be utilized such that a pulsed orCW laser beam is injected into a cavity formed by at least one highlyreflective mirrors or at least one optical element forming a resonantoptical cavity by means of total internal reflection of the light beam.Trace level absorption of a target gas can be detected by utilizing thephoton decay time inside this high-finesse optical cavity. In somevariations, other cavity-enhanced spectroscopy (such as IntegratedCavity Output Spectroscopy (ICOS), Cavity Attenuated Phase ShiftSpectroscopy (CAPS), Cavity Output Autocorrelation Spectroscopy (COAS))will be employed.

Photoacoustic spectroscopy, which is based on the photoacoustic effectcan be utilized. Some of the energy absorbed by target gas moleculeswill result in the rise of gas temperature. Temperature fluctuationswill produce a pressure wave which can be detected by a suitable sensor.By measuring pressure at different wavelengths, a photoacoustic spectrumof the target molecule can be obtained to determine the concentration.

The effects of background changes between measuring the backgroundspectrum without the reactive gas and measuring the unaltered streamspectrum containing reactive gas can be minimized by taking into accountthat background gas compositions can change over time causing the 2 fspectra lineshape to change due to the stream component concentrationchanges and associated broadening effects. In some implementations, ifthe background composition changes during the measurement interval, theresulting line shape variation could lead to measurement uncertaintyresulting from using a direct subtraction between spectra with andwithout the reactive gas present. To minimize such measurementuncertainties, the present techniques can utilize background subtractionmethod comprising: (a) selection of a reference 2 f peak of theinterfering background gas, which does not have interference from thereactive gas, in the same laser scan, (b) measurement of the referencepeak height for both spectra with a reactive gas and spectra without thereactive gas present, (c) scaling of the recorded background 2 f spectrabased on the measured reference peak heights, and (d) determiningreactive gas concentration using the differential signal based on thescaled spectra.

A “curve fitting” method can be utilized to improve detectionsensitivity and measurement repeatability. Even though the reactive gasconcentration can be determined by only relying upon the calibrated 2 fpeak height, the measurement sensitivity and accuracy can in some casesbe limited by noise on the differential 2 f signal. The “curve fitting”method disclosed herein can include recording a reference differentialspectra for a known concentration of the reactive gas, at aconcentration where noise on the 2 f signal is negligible. An H₂Sspecific example is provided for illustration, but it will be understoodthat the general technique is applicable to all reactive gases analyzedusing the current subject matter. FIG. 8 shows the H₂S differentialspectra at different H₂S levels, the curve 802 representing 10 ppmv H₂Sin background gas, which can be used as the reference differentialspectrum used for calibration. A curve fitting method can furtherinclude using this known concentration (for example 10 ppmv in FIG. 8)as a reference and plotting measured differential spectra vs. thereference spectra, since the amplitude of 2 f spectra is proportional tothe gas concentration, relationship between measured differentialspectra and the reference spectra can be fitted well by a linearrelationship as shown in FIG. 9. The reactive gas concentration can bedetermined from the ratio of the slope with respect to the referenceconcentration. This “curve fitting” method can use all or substantiallyall of the information available in the total 2 f-line shape spectrumand greatly improves detection sensitivity and measurementrepeatability. In some variations, the measured 2 f lineshape can bebest fitted directly with theoretical 2 f models for Gaussian,Lorentzian or Voigt lineshapes. The theoretical 2 f models are preferredbut not limited to the models described in one or more of: L. C.Philippe and R. K. Hanson, “Laser Diode Wavelength-ModulationSpectroscopy for Simultaneous Measurement of Temperature, Pressure andVelocity in Shock-Heated Oxygen Flows,” Applied Optics 32, 6090-6103(1993); P. Kluczynski, A. M. Lindberg, and O. Axner, “Background signalsin wavelength-modulation spectrometry with frequency-doubled diode-laserlight I. Theory,” Applied Optics, 783-793, Vol. 40, No. 6, 2001; and H.Li, G. B. Rieker, X. Liu, J. B. Jeffries, and R. K. Hanson, “Extensionof wavelength modulation spectroscopy to large modulation depth fordiode laser absorption measurements in high pressure gases,” AppliedOptics, 45, 1052, 2006, the contents of all of which are herebyincorporated by reference.

As the amount of light absorbed is proportional to the total path lengththe light travels through the sample gas, different absorption cells(such as for example a Herriott cell as discussed above) can be employedto obtain the necessary path length required by the desired sensitivity.The Herriott cell comprises two spherical mirrors spaced at a distancethat enables a certain number of reflections of the laser beam before itmeets the re-entrant condition and goes out of the cell cavity throughthe beam injection hole. With a Herriott cell, long optical paths can beachieved physically compact by reflecting the beam repeatedly withoutinterference between adjacent beams. Depending on the desiredsensitivity, the number of reflections of the Herriott cell can beadjusted by changing the spacing of the two mirrors, by changing thelaser beam injection angle or by using mirrors with different focallengths. Long effective pathlength can also be achieved by usingoff-axis resonating cavity which composes of two highly reflectivemirrors. For calibration purposes, a controlled natural gas and otherbackground gas samples containing a known concentration of the reactivegas can be passed through the absorption cell prior to measurements.

Aspects of the subject matter described herein can be embodied insystems, apparatus, methods, and/or articles depending on the desiredconfiguration. Some implementations of the subject matter describedherein can be realized in digital electronic circuitry, integratedcircuitry, specially designed ASICs (application specific integratedcircuits), computer hardware, firmware, software, and/or combinationsthereof. These various implementations can include implementation in oneor more computer programs that are executable and/or interpretable on aprogrammable system including at least one programmable processor, whichcan be special or general purpose, coupled to receive data andinstructions from, and to transmit data and instructions to, a storagesystem, at least one input device, and at least one output device.

These computer programs (also known as programs, software, softwareapplications, applications, components, or code) include machineinstructions for a programmable processor, and can be implemented in ahigh-level procedural and/or object-oriented programming language,and/or in assembly/machine language. As used herein, the term“machine-readable medium” refers to any computer program product,apparatus and/or device (e.g., magnetic discs, optical disks, memory,Programmable Logic Devices (PLDs)) used to provide machine instructionsand/or data to a programmable processor, including a machine-readablemedium that receives machine instructions as a machine-readable signal.The term “machine-readable signal” refers to any signal used to providemachine instructions and/or data to a programmable processor.

The implementations set forth in the foregoing description do notrepresent all implementations consistent with the subject matterdescribed herein. Instead, they are merely some examples consistent withaspects related to the described subject matter. Wherever possible, thesame reference numbers will be used throughout the drawings to refer tothe same or like parts. Although a few variations have been described indetail above, other modifications or additions are possible. Inparticular, further features and/or variations can be provided inaddition to those set forth herein. For example, the implementationsdescribed above can be directed to various combinations andsubcombinations of the disclosed features and/or combinations andsubcombinations of several further features disclosed above. Inaddition, the logic flow depicted in the accompanying figures and/ordescribed herein does not require the particular order shown, orsequential order, to achieve desirable results. Other embodiments can bewithin the scope of the following claims.

1. A method comprising: determining a first measured concentration of areactive gas in a first sample of a gas mixture, the gas mixturecomprising the reactive gas at a reactive gas concentration and a firstbackground composition that contributes spectral interference thathampers direct spectroscopic measurement of the reactive gasconcentration in the gas mixture, the determining of the first measuredconcentration comprising generating a first differential absorptionspectrum by subtracting a first background absorption data set from afirst sample absorption data set collected for the first sample andconverting the first differential absorption spectrum to a firstmeasured concentration of the reactive gas in the gas mixture usingcalibration data, the first background absorption data set comprisingdata characteristic of absorption characteristics of the firstbackground composition; selecting whether to use the first backgroundabsorption data set or a second background absorption data set todetermine a second measured concentration of the reactive gas in asecond sample of the gas mixture based on whether a correlation of thefirst background absorption data set to the first background compositionis within a pre-defined tolerance; and determining the second measuredconcentration by generating a second differential absorption spectrum bysubtracting either the first background absorption data set or thesecond background absorption data set from the second sample absorptiondata set for the second sample of the gas mixture and converting thesecond differential absorption spectrum to the second measuredconcentration of the reactive gas in the gas mixture using thecalibration data.
 2. A method as in claim 1, further comprising: using asecond background absorption data set that differs from the firstbackground absorption data set to generate the second differentialabsorption spectrum if the first background composition has asubstantially different absorption profile relative to the firstbackground absorption data set; and using a second background absorptiondata set that is identical to the first background absorption data setto generate the second differential absorption spectrum if the firstbackground composition has a substantially similar absorption profilerelative to the first background absorption data set.
 3. A method as inclaim 2, wherein the first differential absorption spectrum comprises afirst region with a first spectral response characteristic of theconcentration of the reactive gas and a second region with a secondspectral response characteristic of a correlation between the firstbackground composition and the first background absorption data set,wherein the first region and the second region do not substantiallyoverlap, and wherein the determining comprises comparing the correlationto a predetermined threshold.
 4. A method as in claim 3, wherein thefirst region comprises wavelengths within approximately ±1 cm⁻¹ of atarget wavelength.
 5. A method as in claim 1, further comprising:collecting the first sample absorption data set using a scannable lasersource operating in a scan range, the scan range comprising a targetwavelength.
 6. A method as in claim 5, wherein the scannable lasersource comprises a tunable diode laser or a quantum cascade laser.
 7. Amethod as in claim 5, wherein the background absorption data set iscollected for a background sample prepared from the gas mixture usingthe scannable laser in the scan range, the preparing of the backgroundsample comprising treating a volume of the gas mixture to reduce aconcentration of the reactive gas without substantially altering thebackground composition in the background sample.
 8. A method as in claim7, wherein the treating of the gas mixture comprises passing the gasmixture through a scrubber material that converts molecules of thereactive gas to a non-gaseous state.
 9. A method as in claim 8, whereinthe scrubber material comprises one or more of metal oxides, cupricdicarbonate, solid and liquid-phase inorganic and organic acids andbases, metal oxide and copper oxide nano particles suspended on largergrain size carrier particles, solid state scrubbers, liquid scrubbers,amine solutions, aqueous ammonia solutions, and aqueous solutions ofstrong acids or bases.
 10. A method as in claim 7, wherein the firstsample absorption data set and the first background absorption data setare collected alternately in a single sample cell.
 11. A method as inclaim 7, wherein the first and second sample absorption data sets andthe first and second background absorption data set are recorded in asample cell and a background sample cell, respectively, withsubstantially identical optical path lengths.
 12. A method as in claim5, wherein the scan range comprises wavelengths within approximately ±2cm⁻¹ or less of the target wavelength.
 13. A method as in claim 12,wherein the reactive gas is hydrogen sulfide and the target wavelengthis selected from the group consisting of 1567.0 nm, 1590.2 nm, 2639.6nm, 1569.9 nm, 1590.6 nm, 2650.1 nm, 1574.6 nm, 1591.1 nm, 3718.7 nm,1576.3 nm, 1601.3 nm, 3730.0 nm, 1578.1 nm, 1919.1 nm, 4049.2 nm, 1581.3nm, 1919.3 nm, 4129.3 nm, 1582.1 nm, 1928.0 nm, 7460.5 nm, 1589.2 nm,1944.6 nm, 7601.7 nm, 1589.8 nm, 2599.1 nm, 7734.6 nm, 1590.0 nm, 2604.7nm, and 7893.3 nm.
 14. A method as in claim 12, wherein the reactive gasis hydrogen chloride and the target wavelength is selected from thegroup consisting of 1727.0 nm, 1785.2 nm, 3395.7 nm, 1730.2 nm, 1793.0nm, 3417.8 nm, 1733.9 nm, 1801.2 nm, 3440.9 nm, 1737.9 nm, 1810.0 nm,3490.3 nm, 1742.4 nm, 1819.1 nm, 3516.6 nm, 1747.2 nm, 3335.4 nm, 3544.1nm, 1752.5 nm, 3354.6 nm, 3572.8 nm, 1777.8 nm, 3374.6 nm, and 3602.6nm.
 15. A method as in claim 12, wherein the reactive gas is hydrogenfluoride and the target wavelength is selected from the group consistingof 1264.3 nm, 1304.5 nm, 2453.8 nm, 1268.3 nm, 1312.6 nm, 2475.9 nm,1273.0 nm, 1321.3 nm, 2499.4 nm, 1278.1 nm, 2395.8 nm, 2550.8 nm, 1283.9nm, 2413.8 nm, 2578.8 nm, 1297.1 nm, 2433.1 nm, and 2608.5 nm.
 16. Amethod as in claim 12, wherein the reactive gas is hydrogen cyanide andthe target wavelength is selected from the group consisting of 1527.4nm, 1844.7 nm, 2483.2 nm, 1528.0 nm, 1845.5 nm, 2484.9 nm, 1528.6 nm,1846.4 nm, 2497.5 nm, 1538.9 nm, 1861.4 nm, 2508.7 nm, 1539.7 nm, 1862.6nm, 2512.6 nm, 1540.5 nm, 1863.7 nm, and 2518.5 nm.
 17. A method as inclaim 12, wherein the reactive gas is hydrogen bromide and the targetwavelength is selected from the group consisting of 1337.6 nm, 1953.9nm, 1995.8 nm, 1339.1 nm, 1957.8 nm, 2002.8 nm, 1341.0 nm, 1962.0 nm,2010.3 nm, 1342.8 nm, 1966.9 nm, 2018.4 nm, 1345.1 nm, 1972.0 nm, 2026.5nm, 1360.4 nm, 1977.1 nm, and 2035.2 nm.
 18. A method as in claim 12,wherein the reactive gas is arsine and the target wavelength is selectedfrom the group consisting of 2162.1 nm, 2201.7 nm, 2851.5 nm, 2165.8 nm,2215.4 nm, 2889.5 nm, 2168.6 nm, 2220.0 nm, 2900.1 nm, 2172.4 nm, 2227.3nm, 2920.6 nm, 2176.4 nm, 2818.4 nm, 2952.3 nm, 2192.2 nm, 2826.5 nm,2975.1 nm, 4555.4 nm, 4582.9 nm 4597.4 nm, 4625.9 nm, 4641.3 nm, 4702.3nm, 4800.4 nm, 4814.9 nm, 4833.2 nm, 4851.9 nm, 4870.8 nm, and 4890.2nm.
 19. A method as in claim 12, wherein the reactive gas is phosphineand the target wavelength is selected from the group consisting of2366.2 nm, 2380.0 nm, 2427.1 nm, 2369.5 nm, 2383.8 nm, 2432.0 nm, 2373.0nm, 2400.3 nm, 2436.9 nm, 2376.4 nm, 2424.8 nm, 2438.4 nm, 4163.4 nm,4187.7 nm, 4200.7 nm, 4215.2 nm, 4226.0 nm, 4241.2 nm, 4301.1 nm, 4311.1nm, 4342.2 nm, 4386.0 nm, 4405.0 nm, and 4431.9 nm.
 20. A method as inclaim 12, wherein the reactive gas is ammonia and the target wavelengthis selected from the group consisting of 1512.0 nm, 1997.2 nm, 2896.4nm, 1531.7 nm, 2005.2 nm, 2897.7 nm, 1955.3 nm, 2230.1 nm, 2912.1 nm,1958.5 nm, 2239.0 nm, 2913.9 nm, 1963.1 nm, 2275.9 nm, 2928.6 nm, 1966.2nm, 2285.2 nm, 2946.5 nm, 1981.5 nm, 2295.7 nm, 2961.8 nm, 1993.3 nm,2305.5 nm, and 2997.3 nm.
 21. A method as in claim 1, wherein the firstand the second background absorption data sets comprise empiricallyobtained historical absorption data for a first and a secondcharacteristic background composition.
 22. A method as in claim 1,wherein the reactive gas is selected from a group consisting of hydrogensulfide, hydrogen chloride, hydrogen fluoride, hydrogen bromide,hydrogen cyanide, arsine, phosphine and ammonia and the targetwavelength is one at which the reactive gas has an absorbance that has afigure of merit greater than 1×10⁻⁶ compared to the backgroundcomposition of the gas mixture.
 23. A method as in claim 1, wherein thebackground composition comprises one or more of a group consisting ofnatural gas, alkanes, alkenes, alkynes, refrigerants, olefins, hydrogen,nitrogen, oxygen, chlorine, carbon dioxide, ammonia, water, carbonmonoxide, hydrocarbons, hydrofluorocarbons, hydrochlorocarbons, andhydrofluorochlorocarbons.
 24. A method as in claim 1, wherein generatingthe first differential absorption spectrum further comprises: selectinga reference spectral peak in the first sample absorption data set, thereference spectral peak being representative of the first backgroundcomposition and having no substantial interference from absorption bythe reactive gas; scaling the first background absorption data set basedon a ratio of the reference spectral peak in the first backgroundabsorption data set and the first absorption spectrum; and subtractingthe scaled first background absorption data set from the first sampleabsorption data set.
 25. A method as in claim 1, wherein generating thefirst differential absorption spectrum further comprises: detecting asecond order or higher harmonic frequency response of light passingthrough the first sample from a scannable laser to a detector, awavelength of the scannable laser being modulated at a modulationfrequency; and converting the second order or higher harmonic frequencyresponse to the first sample absorption data set.